Early efforts to recover slag for end uses outside of the metallurgical environment comprised dumping the slag into an open pit holding a pool of water. The quenched slag would cool and solidify, generating steam that escaped through the slag, thereby producing an "expanded" lightweight product much like natural pumice. The process has the disadvantage of producing copious quantities of gaseous by-products which escape into the atmosphere and it therefore does not comply with modern environmental standards.
It is also slow, being a batch process, and the cooled slag must be treated further to provide an aggregate of uniform size and make it useful.
Processes for pelletizing slag have been developed, in which molten slag is fed continuously with a quantity of water toward a rotary drum having radially-extending vanes, whereby cooled slag is projected into the air for further cooling and, conveniently, is deposited into a heap of discrete, uniform pellets. Such a process is described in Applicant's U.S. Pat. No. 3,594,142. The resulting product is in the form of pellets which are easily handled. However, the pellets which are vitrified and relatively dense, cannot be considered to be a "foamed" slag or an "expanded" slag and they are unsuitable for use in lightweight concrete and similar applications requiring a lightweight aggregate.
In U.S. Pat. No. 1,047,370, molten slag is fed into a stream of water under pressure towards a rotating wheel which carries a multitude of "beater blades" loosely pivoted on their support rods and which stand out radially from the wheel when it is rotated at high speeds of about 7,000 feet per minute. The combined action of steam and water and beaters upon the slag disintegrates the slag into small fragments which are flung through the air and collected in a pile. It is believed that the resulting product does not expand and is unsuitable for use as a lightweight aggregate.
In U.S. Pat. No. 56,595, molten slag is also fed to a rotary drum which, in this case, carries a series of arms extending radially and obliquely around the periphery of the drum. The drum is disposed inside a housing which holds a pool of water to quench hot, molten slag. The slag is granulated and broken by the sudden change in temperature and carried forward by the arms, which operate like an auger, to be scooped and delivered outside apparatus. The resulting product is believed to be vitrified and unsuitable for use as a lightweight aggregate.
An object of this invention is to provide means for producing an expanded, lightweight slag in a continuous process.